1. Field of Invention
The present invention relates to a method of adjusting screen brightness, and more particularly a method of dynamically adjusting screen brightness.
2. Related Art
Since the sensibility of eyes of human beings to brightness changes with the variation of the ambient light level, the screens such as the cathode ray tube screens, liquid crystal screens, plasma screens on which the eyes are focused for a long period of time need to adjust the background brightness in accordance with the ambient light level. When the ambient light level rises, the screen brightness must be increased to fit the changes of sensibility of human eyes to brightness. Meanwhile, the increase of the screen brightness along with the ambient light level may enhance the difference between the screen brightness and the ambient light level, so as to prevent the background brightness of the screen being approximate to or lower than the ambient light level to cause the images on the screen are not easy to identify. On the contrary, when the ambient light level drops, the background brightness of the screen must be decreased simultaneously, so as to prevent the difference between the background brightness of the screen and ambient light level being too great to cause uncomfortable feeling to eyes of human beings focusing on the screens.
FIG. 1 shows a relationship curve of an ambient light level and a brightness adjustment factor in conventional art, which is used for the screen to automatically adjust the background brightness. In FIG. 1, the longitudinal axis is the ambient light level, and the transverse axis is the brightness adjustment factor (display luminance, %) of the screen brightness. In the conventional art, a reference point R is selected and the ambient light level of the reference point R is set to be an original ambient light level Luxoriginal. The corresponding brightness adjustment factor is set to be 100% to serve as a reference. Next, along the longitudinal axis, a longitudinal straight line passing the reference point R is selected to serve as a base line of the brightness adjustment factor. Screen are mostly used in offices, so the original ambient light level Luxoriginal is generally set to be the office brightness, and meanwhile, an appropriate original screen brightness BLKoriginal of the screen is also given to meet the original ambient light level Luxoriginal. Then, a plurality of ambient light levels is obtained and the corresponding screen brightness of the screen under each ambient light level is determined one by one. The screen brightness corresponding to each ambient light level is compared with the screen brightness corresponding to the original ambient light level, so as to obtain the brightness adjustment factors corresponding to all the screen brightness. A corresponding curve of each point is found in the relationship curve in FIG. 1, and this curve is the curve of the screen backlight adjustment factor.
The screen or the computer system for driving the screen uses an optical sensor to continuously monitor the ambient light level. When the ambient light level Luxcurrent is changed to be a new ambient light level Luxnew, a brightness adjustment factor Adjnew corresponding to the ambient light level Luxnew is found, and the obtained brightness adjustment factor Adjnew is multiplied by the current screen brightness BLKcurrent, thus obtaining a new ambient light level BLKnew for the screen.BLKnew=BLKcurrent×Adjnew 
For example, the original ambient light level Luxoriginal is 300 LUX, and the screen brightness is 150 LUX. When the ambient light level is changed to Luxnew=85 LUX, the corresponding brightness adjustment factor Adjnew is 80%. That is, the new screen brightness BLKnew needs to be adjusted to 150×80%=120. Likewise, when the ambient light level is changed to Luxnew=1000 LUX, the corresponding brightness adjustment factor Adjnew is 150%, the new screen brightness BLKnew needs to be adjusted to 150×150%=225 LUX.
However, in the above adjustment method, the brightness adjustment factor is determined by a single and fixed base line, and the brightness adjustment factor corresponding to the original ambient light level is set to be 100% for adjustment. That is to say, the brightness adjustment factor Adj is calculated under the condition that the brightness adjustment factor Adj is the original screen brightness BLKoriginal, but the adjustment is made based on the current screen brightness BLKcurrent.
The current screen brightness BLKcurrent will change along with the adjustment and does not always be the original screen brightness BLKoriginal. Thus, the current screen brightness BLKcurrent will be distorted continuously after several adjustments based on the base line, which results in the background brightness does not fit the change of sensibility of eyes of human beings. For example, when the screen brightness is changed to 150 LUX after several adjustments, the corresponding ambient light level is 75 LUX. When the ambient light level rises to a new ambient light level Luxnew of 85 LUX, the new screen brightness BLKnew should be increased along with the new ambient light level Luxnew in theory. However, in consideration of the base line serving as the reference for the adjustment, the corresponding brightness adjustment factor Adjnew is actually 80%, such that the screen brightness after adjustment is BLKnew=BLKcurrent×Adjnew, i.e., 150×80%=120. That is to say, the new screen brightness BLKnew is decreased instead after the background brightness LuXnew is increased, and thus the screen brightness becomes insufficient and the eyes of human beings cannot clearly see the images on the screen.